JP3199836B2 - Wavelength conversion laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion laser device for converting the wavelength of laser light into higher harmonics using a nonlinear crystal.

[0002]

2. Description of the Related Art In general, a wavelength conversion laser device for converting the wavelength of laser light using a nonlinear crystal has been configured as shown in FIG. That is, reference numeral 1 in the figure denotes a laser oscillator. This laser oscillator 1 has an optical resonator 4 in which a high-reflection mirror 2 and an output mirror 3 are arranged to face each other.
A rod-shaped solid laser medium 5 is disposed in the optical resonator 4 so that its axis is aligned with the optical axis of the optical resonator 4. On the side of the solid laser medium 5, there is an excitation means.
An excitation lamp 6 is arranged in parallel. Further, in the optical resonator 4, a mode locker 7 for causing a mode rocking between the solid-state laser medium 5 and the output mirror-3 is provided.
Is arranged. An acousto-optic device, a supersaturated dye, or the like is used as the modrocker 7.

When the solid laser medium 5 is excited by an excitation lamp 6, laser light P can be output from the output mirror-3. Here, assuming that the optical resonator length is L and the speed of light is C, the frequency f is f = C / 2L, and the pulse width is the reciprocal (Δν ⁻¹ ) of the gain width Δν of the solid-state laser medium 5. A laser output of a short pulse train of the order is obtained.

The laser beam P of the fundamental wavelength output from the output mirror 3 passes through an output modulator 8 and is partially split by a first beam splitter 9. The laser beam S transmitted through the first beam splitter 9 is focused by a condenser lens 11 and is incident on a nonlinear crystal 12 such as SHG. The nonlinear crystal 12 converts the laser light S of the fundamental wavelength output from the optical resonator 4 into a harmonic having a wavelength of half. Since the laser light S emitted from the nonlinear crystal 12 contains a fundamental wave in a harmonic, the laser light S transmits the harmonic reflected by the second beam splitter 13 and the harmonic. It is separated into fundamental waves.

A part of the laser beam S separated by the first beam splitter 9 has its intensity detected by a photodetector 14. The detection signal is input to the output modulator 8. Thereby, the intensity of the laser light S is controlled, so that the intensity of the laser light S incident on the nonlinear crystal 12 is kept constant. That is, although the laser output becomes unstable due to various factors, the output is detected by the photodetector 14, and
The output signal is fed back to the output modulator 8 to stabilize the output.

In order to stabilize the output as described above, a high-speed response is desired. Therefore, an acousto-optic device is generally used as the output modulator 8. However, its response is at most about 100 kHz. Optical resonator length is 2
The laser mode lock frequency of about m is f = C / 2L =
(3 × 10 ⁸ ) / (2 × 2) = 75 MHz. Therefore, the frequency is too large compared to the response of the acousto-optic element, and the output cannot be stabilized for each pulse.

In addition, when SHG is used as the nonlinear crystal 12, its output is the laser incident on the nonlinear crystal 12.
Since it is approximately proportional to the square of the intensity of the light S, for example,
The stability of the output of the light S is ± 10% as shown in FIG.
%, The intensity of the harmonic output from the nonlinear crystal 12 is expanded to about ± 20% as shown in FIG. 6B. Therefore, it may not be possible to use the laser light S in applications where it is necessary to stabilize the intensity of each pulse.

[0008]

As described above, since the output from the nonlinear crystal used to convert the wavelength into a harmonic is proportional to the square of the intensity of the laser light incident thereon, the laser -If the intensity of the laser light output from the oscillator fluctuates even slightly, the intensity fluctuation of the harmonic output from the nonlinear crystal may increase.

The present invention has been made based on the above circumstances, and an object of the present invention is to make it possible to obtain a harmonic output from a nonlinear crystal even when the intensity of laser light output from a laser oscillator fluctuates. Wavelength conversion that can stabilize the intensity of light
It is to provide a laser device .

[0010]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a laser oscillator comprising an optical resonator, a laser medium provided in the optical resonator, and the laser oscillator. A nonlinear crystal for converting the wavelength of the laser light output from the laser into a harmonic, and an intensity of the laser light output from the laser oscillator provided between the nonlinear crystal and the laser oscillator. And a self-focusing element that changes the beam diameter of laser light incident on the nonlinear crystal according to the focal length.

[0011]

According to the above arrangement, the intensity of the laser light incident on the nonlinear crystal can be changed according to the variation in the intensity of the laser light output from the laser oscillator. The intensity of the emitted harmonic can be stabilized.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. The same parts as those in the configuration shown in FIG.

That is, in the embodiment of the present invention,
The laser light S output from the laser oscillator 1 and having its intensity controlled by the output modulator 8, the first beam splitter 9, and the photodetector 14 is fed back to the self-focusing element 22.
Incident on. That is, the self-focusing element 22 is disposed between the laser oscillator 1 and the nonlinear crystal 12.

The self-focusing element 22 has the property of changing the focal length for focusing the laser light S in accordance with the intensity of the laser light S incident thereon. That is, the intensity of the laser light S incident on the self-focusing element 22 is P _{1 in} FIG.
A strong focal distance when, as shown in is assumed to be F _1, les as shown by P ₂ in FIG. 2 (b) - when the intensity of the laser light S is decreased, Le - said focusing the laser light S the focal length of the self-focusing element 22 becomes longer distance F ₂ than the F _1. The non-linear crystal 12 is arranged at a position farther than the focal point F _{1 and} on the focal point F ₂ with its center substantially coincident. Examples of the substance having a nature of such self-focusing, CS ₂ solution is known. Therefore, as the self-focusing element 22, one in which the CS ₂ liquid is filled in a container may be used. The self-focusing element 2
2 Galle - focal position for focusing the laser light S can be varied depending on the thickness and arrangement position of the vessel CS ₂ solution is accommodated.

On the other hand, ^assuming that the intensity of the harmonic output from the nonlinear crystal 12 is P ^2w , the intensity P ^2w is the intensity I 2 of the excitation light emitted from the excitation lamp 6 for exciting the solid-state laser medium 5. ^w And the intensity P ^{w of the} laser light S output from the laser oscillator 1 Is proportional to That is, (P
^{2w to} I ^w ・ P ^w ). Therefore, for example, the intensity I ^{w of the} excitation light Decreases, the intensity P of the laser light S
^w Increases, and the harmonic intensity P ^2w is kept almost constant.

Next, the operation of the wavelength conversion laser device having the above configuration will be described. Figure 2 Les enters the self-focusing element 22, as shown in (a) - if the intensity of laser light S is strong and P ₁ is the focal length of the self-focusing element 22 becomes F _1. Therefore, the laser light S of the fundamental wavelength emitted from the self-focusing element 22 is focused on the near side of the nonlinear crystal 12, and then enters the nonlinear crystal 12 in a divergent state. If the laser light S is incident on the nonlinear crystal 12 in a divergent state, the beam diameter of the laser light S on the nonlinear crystal 12 is enlarged, and therefore the nonlinear crystal 12 receives a unit area per unit area. The laser strength will be reduced.

As described above, when the intensity of the laser beam S incident on the nonlinear crystal 12 per unit area decreases, the intensity of the harmonic converted by the nonlinear crystal 12 and outputted becomes P.
It will be lower than ₁ . The intensity of the harmonics of this time is P _3.

On the other hand, the laser output from the laser oscillator 1
When the intensity of the laser light S is lowered to the P ₂ as shown in FIG. 2 (b) variation of the intensity P ₂ les - by laser light S is incident on the self-focusing element 22, the focal point the distance is changed to F _2. That is, since the focal length F ₂ of the self-focusing element 22 is longer than the case of FIG. 2 (a), the focused les -
The light S will be focused on the nonlinear crystal 12. As a result, the beam diameter of the laser beam S incident on the nonlinear crystal 12 becomes smaller than that in the case of FIG. 2A, so that the laser intensity per unit area which the nonlinear crystal 12 receives increases. .

Therefore, the intensity of the harmonic output after being wavelength-converted by the nonlinear crystal 12 is higher than P ₂ . If the arrangement of the self-focusing element 22 and the nonlinear crystal 12 is adjusted, the nonlinear crystal 1 shown in FIG.
The intensity of the harmonics output from the 2 may be the same as the intensity P ₃ when Figure 2 (a).

That is, even if the intensity of the laser light S output from the laser oscillator 1 fluctuates, the focal length of the laser light S incident on the nonlinear crystal 12 can be changed by the self-focusing element 22. Thus, the intensity of the harmonic output from the nonlinear crystal 12 can be made constant. In addition, since the self-focusing element 22 responds at high speed, the frequency is 100 MHz.
The present invention can also be applied to a mode lock laser of about z.

FIGS. 3A to 3D show the present invention and the conventional technique.
The comparison with the operation is shown. The solid line in the figure indicates the present invention,
The dashed line indicates the conventional technology . FIG. 1A shows a laser oscillator 1.
The relationship between the intensity of the laser light S output from the lens and the focal length is constant. Since the light was focused by the condenser lens 11 in the related art, it is constant. As the intensity of the laser light S increases, the focal length decreases.

FIG. 2B shows the intensity of the laser beam S output from the laser oscillator 1 and the laser beam incident on the nonlinear crystal 12.
The relationship between the laser beam S and the beam diameter is shown. The conventional beam diameter focused by the condenser lens 11 is constant. The beam diameter increases as the strength increases.

FIG. 3C shows the intensity of the laser beam S output from the laser oscillator 1 and the laser beam incident on the nonlinear crystal 12.
The relationship with the intensity of the light S, that is, the excitation intensity of the nonlinear crystal 12 is shown. In the prior art shown by the dashed line , the intensity of exciting the nonlinear crystal 12 increases as the intensity of the laser light S increases. However, according to the present invention, when the intensity of the laser light S increases, the nonlinear crystal 12 , The intensity of the excitation decreases.

FIG. 3D shows the relationship between the intensity of the laser light S output from the laser oscillator 1 and the intensity of the harmonic output from the nonlinear crystal 12. Conventionally, as the intensity of the laser light S increases, the intensity of the harmonic output from the nonlinear crystal 12 also increases. However, according to the present invention, the intensity of the harmonic output from the nonlinear crystal 12 is made substantially constant. can do.

FIG. 4 shows another embodiment of the present invention. In this embodiment, the correction lens 31 is provided on the emission side of the self-focusing element 22. According to such a configuration, the focal length of the laser light S emitted from the self-focusing element 22, that is, the nonlinear crystal The beam diameter of the laser beam S incident on the lens 12 can be corrected by the correction lens 31.

The present invention is not limited to the above embodiments, but may be applied to lasers other than a mode lock laser such as a normal pulse and a Q switch. Further, the non-linear crystal may be a crystal other than SHG, for example, a crystal that converts into a third harmonic or a fourth harmonic. In the case where the intensity of harmonics is stabilized by using a self-focusing element, an output modulator for controlling the intensity of laser light output from a laser oscillator may not be provided.

[0027]

As described above, according to the present invention, the focal length changes between the laser oscillator and the nonlinear crystal in accordance with the intensity of the laser light output from the laser oscillator, and the nonlinearity increases. A self-focusing element for changing the beam diameter of the laser light incident on the crystal is provided.

Therefore, even if the intensity of the laser light output from the laser oscillator fluctuates, the intensity of the laser light incident on the nonlinear crystal can be made substantially constant. The intensity of the higher harmonic can be kept constant.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an embodiment of the present invention.

2A is an explanatory diagram when the intensity of laser light incident on a self-focusing element is high, and FIG. 2B is an explanatory diagram when the intensity is low.

FIGS. 3A to 3D are explanatory diagrams showing the relationship between the intensity of laser light output from a laser oscillator according to the related art and the present invention and various parameters.

FIG. 4 is an overall configuration diagram showing another embodiment of the present invention.

FIG. 5 is an overall configuration diagram of the related art.

6A is an intensity distribution diagram of laser light output from a laser oscillator, and FIG. 6B is an intensity distribution diagram of laser light output from a conventional nonlinear crystal.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Laser oscillator, 4 ... Optical resonator, 5 ... Solid laser medium, 12 ... Non-linear crystal, 22 ... Self-focusing element.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-141631 (JP, A) JP-A-63-148242 (JP, A) Soviet Journal of Quantum Electronics, Vol. 8, No. 2, pp. 272-273 (1978) Yu. G. FIG. Grin ', Yu. N. Karamzin and A. P. Sukhorukov (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/37-1/39 H01S 3/108-3/109 INSPEC (DIALOG) JICST file (JOIS) WPI (DIALOG)

Claims (1)

(57) [Claims] 1. A laser oscillator comprising an optical resonator and a laser medium provided in the optical resonator, and converting a wavelength of laser light output from the laser oscillator into a harmonic. A nonlinear crystal which is provided between the nonlinear crystal and the laser oscillator, and whose focal length changes according to the intensity of laser light output from the laser oscillator, and which is incident on the nonlinear crystal. A wavelength conversion laser device comprising: a self-focusing element for changing a beam diameter of the light.